1. Field of the Invention
The present invention relates to coating compositions based on polyurethane polymers and formaldehyde resins wherein at least a portion of the chain lengthening agent used to prepare the polyurethane prepolymer is 2,2-bis(hydroxymethyl)propionic acid amide. The present invention is also directed to the preparation of these coating compositions and their use as an adhesive coat in the direct or reverse coating of textiles.
2. Description of the Prior Art
The prior art has known for some time to coat textiles, such as woven cloth, worked cloth or webs with solutions of polyurethanes according to the direct or reverse coating process to produce synthetic leather and similar articles. The coating solutions may either contain urethane prepolymers which are reacted with polyfunctional cross-linkers to attain their final plastics properties and product properties (so-called two-component polyurethanes) or high molecular weight, fully reacted polyurethanes which already fully have their plastics properties (so-called one-component polyurethanes). The coatings which are thus obtained are used for the production of outer garments, leather bags, shoe upper material, tarpaulins, canvasses, upholstery and many other articles.
It is known from DE-OS No. 2,814,173 to combine prepolymers which contain urethane groups and have from 2 to 8 terminal OH--, --CONH.sub.2 and/or ketoxime urethane groups with urea- or melamine-formaldehyde to produce a cross-linkable mixture. These products only attain their high molecular weight after the cross-linking reaction and prior to this have no mechanical strength. It is, however, often advantageous and desirable for coating compositions to have a certain degree of mechanical strength even before their passage through a drying channel.
It is, moreover, known from DE-OS No. 2,457,387 that specific polyurethane products based on aliphatic or cycloaliphatic polyisocyanates and hydrazine as the chain lengthening agent which are cross-linked with formaldehyde resins, have an advantageously soft, nappa leather feel. These coating compositions are, however, unsuitable for all uses.
The particular disadvantage of all known polyurethanes based on dihydroxy polyethers and/or polyesters and diisocyanates with diols and/or polyols as chain lengthening agents and which are hardened with melamine- and/or urea-formaldehyde resins, is the relatively high curing temperature (about 170.degree. C.) which is required.
It is an object of the present invention to provide PUR-coating compositions, preferably in the form of solutions, which may be cured with formaldehyde resins at a relatively low temperature in a short period of time.
It has now been found that coating compositions can be produced which may be adequately cross-linked with formaldehyde resins at relatively low temperature and with a relatively short cross-linking time, if 2,2-bis(hydroxymethyl)propionic acid amide is incorporated into the polyurethanes as the sole chain lengthening agent or in admixture with other chain lengthening agents.
2,2-bis(hydroxymethyl)propionic acid amide is known from DE-A No. 2,631,284 or DE-A No. 3,107,060. DE-A No. 2,631,284 describes the use of this substance as a skin moisturizing agent for cosmetic products. DE-OS No. 3,107,060 explains how to use the said compound to prevent the turbidity of a control or standard serum. An earlier use of the said amide in polyurethane chemistry in particular for PU-coating compositions which may be easily cross-linked by heat is not known.